Share tags2

Supersedes #11351.
Not sure if this and `getTagsForUser()` are that useful to the outside, though -- if you want a list of tags that are relevant for a given user (i.e. owned by him, and shared with him if ou have `$includeCollections=true`), you can use `getTags()` like before.
I think I'd rather not put them into the interface for now; if need arises, it's easy to add them.